u32 ssthresh(struct sock *sk); u32 cong_avoid(struct sock *sk, u32 ack, u32 rtt,
u32 in_flight, int good_ack); u32 min_cwnd(struct sock *sk);. Optionally
implement.
Pluggable TCP Congestion Avoidance Modules for eVLBI
JOINT INSTITUTE FOR VLBI IN EUROPE
Mark Kettenis, Arpad Szomoru Joint Institute for VLBI in Europe
[email protected]
Outline Introduction C-TCP Pluggable Congestion Avoindance Modules A simple module for eVLBI Measurements Conclusions
Datatransport for eVLBI at JIVE Boundary conditions: Mark4 correlator Real time Which means: Correaltion within a second after observation Timely delivery of data is important Exceedingly more difficult at higher data rates Packet loss increases Fixed size hardware buffers so timing gets more critical Power-of-two datarates
Datatransport for eVLBI at JIVE Mark4 datarates 128Mb/sRoutine for science observations 256Mb/sPossible in most "weather" conditions 512Mb/sIf we get lucky 1Gb/sImpossible with std 1Gb/s ethernet Bandwidth seems to be available, yet we can’t always transfer data at 512Mb/s.
Using TCP for eVLBI Probably not the optimal choice exponential backoff retransmission Unfortunately... Mark5A UDP doesn’t really work
C-TCP Developed by CHEETAH project for circuit-switched networks Patches to Linux 2.6.11 that Disable slow start completely Fix the congestion window at the bandwidth delay product
For connections marked by userland app Problems Needs web100 Not available for other kernels
Benefits Application control possible http://cheetah.cs.virginia.edu/
Linux Pluggable Congestion Avoidance Modules Minimally implement u32 ssthresh(struct sock *sk); u32 cong_avoid(struct sock *sk, u32 ack, u32 rtt, u32 in_flight, int good_ack); u32 min_cwnd(struct sock *sk);
Optionally implement u32 rtt_sample(struct sock *sk, u32 usrtt); u32 undo_cwnd(struct sock *sk); And more...
Access to "struct sock" with all details about the connection destination address & port source address & port
Linux Pluggable Congestion Avoidance Modules Benefits Only the sender needs to be modified No need to reboot...but don’t lock yourself in. Stable interface? Disadvantages No application control Must run Linux 2.6.13 or later Documentation missing?
A simple module for eVLBI Based on the ideas from C-TCP Very small slow start threshold Fix congestion window
Only activated when destination is Mark5A data port "Standard" Reno for other connections Bandwidth specified when module is loaded Less than 50 lines of code
Measurements Bandwidth measurements using iperf between Amsterdam and JIVE Mark5’s directly connected to 1Gb/s links Tests between two Mark5s With "background" traffic from a third Mark5 Conclusions Aggressive Congestion (non)-Avoidance can give an improvement Connectivity provided by SURFNet is too good The Netherlands is too small for meaningful tests
Measurements Bandwidth measurements using iperf between Medicina and JIVE
Measurements Bandwidth measurements using Mark5A failed Stable version of Mark5A code doesn’t support Linux 2.6.13 Development version seems to be broken Datatransport up to 256Mb/s seemed to work
Conclusions Pluggable modules provide interesting flexability Fluctuations in throughput are smaller Evaluation for real eVLBI remains to be done Application level changes needed
Thanks
Many thanks to Guiseppe Maccaferri Radiotelescopi di Medicina Istituto di Radioastronomia
